An electro-optic modulator is a device that employs the electro-optic effect to modulate a beam of light. The electro-optic effect refers to the modification of a medium's refractive index which is caused by subjecting the medium to an electric field. In many common electro-optic modulators, the medium is a crystal that is positioned between a capacitor. A varying voltage is applied to the capacitor to vary the electric field experienced by the crystal thereby altering the refractive index of the crystal. As a result, as a beam of light (e.g. a laser) passes through the crystal, the variations in the refractive index cause the phase of the beam of light to be modulated. Also, to modulate the amplitude of a beam of light, an electro-optic modulator can include a Mach-Zehnder interferometer or other similar device.
It is common to use return-to-zero phase shift keying to modulate an optic signal. For example, return-to-zero differential phase shift keying (RZ-DPSK), return-to-zero binary phase shift keying (RZ-DPSK), return-to-zero quadrature phase shift keying (RZ-QPSK) could be used. Pulse position modulation may also be used.
FIG. 1 illustrates an example of a prior art circuit 100 for modulating an optic signal using RZ-DPSK. Circuit 100 employs two electro-optic modulators. First, as an optic signal is passed through a phase modulator, modulator 101, a data signal is input to modulator 101 to modulate the optic signal with the data. Then, the optic signal is passed through an amplitude modulator, modulator 102, where it is amplitude modulated with a clock giving the optic signal its return-to-zero characteristic. Three modulator architectures also exist to apply amplitude modulation at different clock rates for bursted RZ-DPSK waveforms.
The architecture of circuit 100 can be undesirable for many implementations. For example, because modulators 101 and 102 are relatively large components, the overall size of a system that employs circuit 100 can be unsuitable. The use of two modulators also adds cost to the system and reduces its reliability. Also, because two separate modulators are employed, there are stringent skew requirements to ensure that the clock is modulated in alignment with the previously modulated data.